In this article, we demonstrate a novel micro optical viscosity sensor (MOVS) based on a laser-induced capillary wave with a focus control system that enables in situ monitoring of viscosity and surface tension changes in microliter-order liquid samples such as body fluids, polymer coating materials, lubricants, heavy oils and so on. The microfabricated sensor consists of two deep trenches (depth of 273 µm) holding photonic crystal fibers (PCFs), and three shallow trenches (depth of 125 µm) holding collimating lensed fibers (CLFs) for the probing laser. The capillary wave is excited by two pulsed laser beams generating optical interference, and the rapid motion of the capillary wave, which contains information regarding the viscosity and surface tension of the sample, is monitored by detecting the first-order diffracted beam of the probing laser irradiated onto the sample surface. In order to apply this sensor in manufacturing and clinical settings, the distance between the liquid level and the sensor must be properly adjusted because the sample surface is strongly influenced by evaporation and outside vibration disturbances under such conditions. In the MOVS, the specular reflection of the probing laser is detected by a symmetrically placed collimating lensed fiber. Maximizing the signal of specular reflection by using a piezo stage connected to the MOVS and PID controller, the focal points of the fibers (PCFs and CLFs) are adjusted on the sample surface. The high-reproducible measurement results under evaporation and outside disturbance indicate the validity of MOVS for in situ application.

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